1. Field of the Invention
The present invention relates to a digital FM demodulation apparatus demodulating FM modulated waves, and particularly to a digital FM demodulation apparatus performing digital sampling at a high frequency.
2. Description of the Background Art
While an FM modulated wave has been demodulated conventionally by an analog FM demodulation apparatus, an FM demodulation apparatus has been developed with a digital circuit which has been made more functional due to advances in semiconductor technology.
A conventional digital FM demodulation apparatus performs digital FM demodulation in an arc tangent system, description of which will hereinafter be described.
An FM modulated wave y(t) is described by: EQU y(t)=sin.phi.(t) (1)
where .phi.(t) is a carrier phase. The carrier phase .phi.(t) should satisfy the following relation with a demodulated wave Y(t): EQU d.phi.(t)/dt=.omega.(t)=.omega..sub.c +Y(t) (2)
The above equation shows change of an instantaneous frequency .omega. of the carrier in the vicinity of .omega..sub.c according to Y(t).
Considering the carrier, as two components A and B orthogonal to each other, the following equation is obtained: EQU tan.phi.(t)=B/A (3)
From the equations (2) and (3), the demodulated wave Y(t) is described by the following: EQU Y(t)=d {atan (B/A)}/dt (4)
With a hardware structured according to the above equations, a digital FM demodulation apparatus in an arc tangent system can be implemented.
FIG. 7 is a block diagram of a conventional digital FM demodulation apparatus in accordance with the above-described system. As shown in FIG. 7, the digital FM demodulation apparatus includes a sampling circuit 11, a 90.degree. phase splitter 12, an arithmetic circuit 13, and a differentiating circuit 14. 90.degree. phase splitter 12 splits an FM modulated wave y(t) sampled by sampling circuit 11 into a component A with respect to 0.degree. and a component B with respect to 90.degree. to provide the same to arithmetic circuit 13. Arithmetic circuit 13 includes a table ROM (Read Only Memory) in which a value of atan(B/A) to be defined by the components A and B of the FM modulated wave y(t) has been written. Arithmetic circuit 13 reads out the value of atan (B/A) to be defined by the components A and B, i.e. a carrier phase .phi.(t), from the table ROM to provide the same to differentiating circuit 14. Arithmetic circuit 13 does not carry out an operation of atan (B/A), but only reads out a value from the table ROM. Accordingly, the processing requires less time, which will scarcely cause delay. Differentiating circuit 14 differentiates the input carrier phase .phi.(t) to provide a demodulated wave Y(t), and FM demodulation is thus completed.
FIG. 8 is a detailed block diagram showing 90.degree. phase splitter 12 in the digital FM demodulation apparatus. Since the circuit shown in FIG. 8 is a phase circuit employing Hilbert transform constituted of delay, multiplication, and addition and subtraction, various delay circuits, multiplier circuits, and adder-subtractor circuits are required, causing large scale and long delay time of the circuit.